In prior constructions the combustion chambers of turbojet engines, whose section perpendicular to the direction of the gaseous flow is approximately annular or circular, are contained within a housing having intake vents for air supplied by the compresser. The space inside the chamber shell, called the flame tube, contains the fuel injection devices and the primary combustion air intake vents at its front (upstream) end and similar vents for admitting the post-combustion and/or dilution air at its rear (downstream) end.
Since the inner surface of the wall or walls of the chamber shells are subjected to an intense heat flow generated by the radiation from the flames and the convection of the combustion gases flowing from front to rear, it is advantageous to utilize a portion of the secondary air flow, that is, of the air flow which fulfills functions other than combustion, to effect the thermal protection of the said walls. Such protection by a portion of the secondary air flow may be achieved by two procedures which, moreover, can often be used simultaneously to advantage. The first of these procedures calls for cooling by external convection; that is, the heat of the wall is dissipated by the flow of secondary air flowing from front to rear around its outer surface. The second, often called "film cooling", calls in addition for the heat-shield effect; air vents or intake ports in the walls distribute a portion of the secondary air flow along their inner surface in the form of a surface layer which prevents direct contact between the surface and the combustion gases. This layer, which becomes diluted as it progresses from front to rear, must be renewed by means of intakes of secondary air successively distributed throughout the length of the chamber.
The high compression at which modern jet turbines operate has significantly complicated the problem of cooling the chamber shell walls, since, as compression increases, so does the intake temperature of the secondary air (600.degree. C for a compression ratio of 30) and the thrust of the combustion gases, which is a function of their pressure.
Several approaches are known for improving the protection of the walls.
The first consists of multiplying the number of surface air inlets, so as to increase the flow of such air and renew it more frequently. This approach is limited by the fact that the flow of secondary air cannot exceed a fraction equal to 30% to 35% of the total flow; otherwise, the flow of primary air, whose relative value must increase in proportion to the turbine intake temperature -- which has been rising in the present trend -- becomes insufficient, and so, moreover, does the flow of secondary dilution air.
Another approach consists of accelerating the heat exchange by convection by increasing the speed, and thus the flow rate, of the portion of the flow of secondary air which goes around the chamber. This rate of flow is quickly limited, however, by increased pressure losses and the impossibility of correctly feeding intake vents of very large dimensions.
Still another approach, which also is intended to accelerate the exchange of heat by convection, consists of enlarging the exchange surfaces. One may, for example, equip the outer surfaces of the walls with fins, but the construction of such fins becomes difficult. One may also build the chamber shell out of double-walled sections, that is, wherein the wall or walls consists of two facings and the facings of each wall ection are joined to each other by ribs which form braces aligned in the direction of the flow and delimit longitudinal channels opening into the space between the housing and the chamber at the front and into the chamber at the rear, so that the same current of air flows first through the channels to cool one section by convection and then along the inner face of the following section to cool it by "film cooling". However, the double-wall arrangement requires very large air flows in order for a significant convection effect to be obtained.
All the approaches described above to achieve convection cooling present a major drawback: the convective fluid flows from the front to the rear and gets hot along its path in contact with a wall whose temperature itself increases from front to rear because of the progressive dilution of the film cooling effect.